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This study investigates the relative contribution of internal variability and external forcings on summer (June–August) surface air temperature (SAT) over the central Indian landmass. Here we use Community Earth System Model Large Ensemble (CESM‐LE) data to assess the historical (1966–2005) and future (2010–2060) climate change in presence of internal climate variability. The summer SAT trend during the historical period exhibits an amplified cooling (<−3°C), whereas a warming trend (≥4°C) is projected in all the ensemble members under RCP8.5 scenario. The total trend is then partitioned into contributions from the externally forced response and internal climatic variability. Over the Indian region, the external forcing displays a strong cooling trend during the historical period and warming trend under RCP8.5 scenario. On the other hand, natural variability displays mainly cooling trends and it introduces a wide range of uncertainty to the future projection in climate models. In historical period, the signal‐to‐noise ratio (SNR), that is, ratio of external forcings and internal variability, is less than 1, which indicates that the internal climatic variability dominates over the forced response. But in future decades the SNR is much higher than 1, that is, external forcing overrides the internal variability. However, to a greater extent natural variability will mask the warming trend over the Indian region, even under RCP8.5 scenario. [ABSTRACT FROM AUTHOR]

Using the Community Earth System Model (CESM)-Large Ensemble (LE) surface air temperature (SAT) data, we investigate the multidecadal changes in SAT variability over Central Indian landmass, particularly the Indo-Gangetic (IG) river basin. This region comes under the active influence of the Indian summer monsoon, and during the summer monsoon months (JJA), we observe an amplified cooling (< − 3 °C) trend (1961–2000) in SAT. This SAT trend is considered as a superposition of external forcings and natural climatic variability. The forced response is computed by averaging the trend in 35 ensemble members, which displays a moderate cooling trend due to aerosol-, ozone-, and volcano-only forcings. But the internal variability introduces a wide range of uncertainties in SAT, with majority of the members display a strong cooling trend in the Central Indian region. During the entire period, natural climatic variability dominates over the forced response, which strongly overrides the greenhouse gas (GHG) warming. Here, we separate out the influence of global climate variability on regional climate variability and identify the specific internal variability which is responsible for the multidecadal cooling trend in the analyzed region. Furthermore, we investigate the specific physical mechanism driving the cooling trend and analyze the role of Atlantic multidecadal oscillation (AMO) in its negative phase. The covariability is − 0.74, i.e., AMO accounts for ~ 55% of total variance in the multidecadal variability. In the negative phase of AMO, strong signals of Rossby waves emanating from North Atlantic Ocean propagate across the Eurasian continent, and in the latter half of the twentieth century, the effect of this cold sea surface temperature (SST) anomaly is felt in the Central Indian landmass (particularly over IG river basin) through teleconnection. This study will increase the predictability of multidecadal variability in SAT during summer monsoon season over the Central Indian region with AMO as a strong driving component. [ABSTRACT FROM AUTHOR]

During 1961−2000, the CRU based summer surface air temperature (SAT) exhibits a significant cooling trend (~−3°C/40 yrs.) in the Humid Subtropical Climate Zone, India. Here we investigate the contribution of internal and external factors, which are driving this cooling trend. Using the Community Earth System Model-Large Ensemble (CESM-LE), we analyze the historical climate change in presence of internal climate variability. The summer SAT trend during the historical period exhibit an amplified cooling (<−3°C), whereas, a warming trend (>= 4°C) is projected in all the ensemble members under RCP8.5 scenario. The total trend is partitioned into contributions from the externally forced response and internal climatic variability. Further analyses reveals that external forcing displays a cooling and warming trend over the region under historical and RCP8.5 scenarios, respectively, while internal variability displays mixed cooling (in most cases) and warming signals. Therefore, the internal variability introduces a wide range of uncertainty to the future projection in climate models. In historical period, the signal to noise ratio is less than 1, which indicates that the internal climatic variability dominates over the forced response. But in future decades the SNR is much higher than 1. However, to a greater extent internal variability will mask the warming trend over the region, even under RCP8.5 scenario. Furthermore, to quantify the role of different external forcing factors we used the CCSM4 single forcing simulations for the historical period. The simulation results from CESM-LE and CCSM4 suggest that the cooling trend over the region is primarily due to the combined influence of internal variability (~73%) and partly due to aerosol (~10%) and ozone only forcing, which strongly mask the warming effect of GHG and solar forcing. [ABSTRACT FROM AUTHOR]